Typically, in order to reduce an amount of a vehicle roll that occurs during a vehicle cornering, a vehicle such as an automobile is provided with a stabilizer that is formed by conducting a hot-bending process on a spring steel material or the like (see Document 1).
Such a stabilizer has a complicated configuration in which a plurality of points at a lengthwise intermediate portion of the stabilizer is bent in order to avoid the stabilizer from interfering with a lower portion and a suspension of a vehicle body when the stabilizer is attached to a vehicle. However, generally, the stabilizer comprises a torsion portion that is existed to extend in a vehicle widthwise direction in a state of being attached to the vehicle and arm portions extending respectively from both left and right end portions of the torsion portion in a front and back direction of the vehicle. Further, a continuous portion spanning between the torsion portion and each of the arm portions is formed into a circular arc shape and is provided with a bending portion.
Here, the torsion portion is connected at the vehicle body side through a rubber bush, a bracket or the like. Link members are also connected to tip end portions of the arm portions by using bolts or the like, and the arm portions are connected, via the link members, to each member at a side of an axle such as a suspension arm or the like. Then, during a vehicle cornering, vertical loads in directions opposite to each other are applied to the tip end portions of the respective arm portions so that the respective arm portions are deflected in directions opposite to each other, and the torsion portion is twisted due to a moment. Accordingly, this stabilizer is structured to allow a resilient reaction force to act on the member at the axle side and, suppress the roll of the vehicle body.
Therefore, high load is repeatedly applied to such a stabilizer, which causes a problem with durability such as fatigue life. In particular, a maximum stress that is applied to the stabilizer tends to generate on the bending portion. However, generally, due to contacting with a mold during the hot-bending process, a flat portion, what is called, a “tool mark” is formed at the bending portion, therefore, the cross section of the bending portion is made flat to form a substantially D-shape. For this reason, when the stabilizer is actually attached to a vehicle for practical use, stress is often concentrated on the “tool mark” of the bending portion, which often causes the stabilizer to be broken due to the concentration of stress. Accordingly, in order to improve the durability of the stabilizer, fatigue life of the bending portion must be maintained as long as possible.
In particular, due to an increase of family of a vehicle having a large vehicle height such as a so-called SUV (sports utility vehicle), high roll rigidity of the stabilizer is highly required. Further, in order to obtain a crushable zone in the vehicle, it is also required that arm portions should be made as short as possible. For this reason, there is a tendency that stress applied to the-bending portion increases more and more. Consequently, in such a stabilizer used under high stress as described above, further improvement in durability is required. (Document 1: Japanese Patent Application Laid-Open (JP-A) No. 7-215038)